1. Technical Field of the Invention
The present invention relates to a projecting display apparatus for projecting an image of an image display panel onto a screen.
2. Background of the Invention
A rear projection display is provided with a cabinet having a screen, a plane mirror provided to incline to a rear side of the screen, and a projector engine for emitting image light to the plane mirror and an image projected to the rear side of the screen is enjoyed from a front side thereof. The projector engine is provided with an illuminating optical system for producing illuminating light having a uniform illuminance from a white color light source having a high brightness, an image display panel for modulating the illuminating light into image light, a projecting optical system for enlarging the image light to focus on the screen. According to a rear projection display of a spreading type, as the image display panel, the main stream is constituted by a three plate type using three sheets of liquid crystal panels of LCOS or the like and a single plate type using one sheet of a digital micromirror device (DMD).
According to the projecting display apparatus of the single plate type having DMD constituting the image display panel of a reflection type, a total reflection prism (TIR prism) is provided between the illuminating optical system and the projecting optical system. The total reflection prism is used as means for splitting a light path of illuminating light and a light path of image light by restraining loss of a light amount. The total reflection prism is constituted by, for example, combining two triangular prisms and an air gap is provided between the two triangular prisms in order to promote an efficiency of total reflection operation.
According to a projecting display apparatus described in JP-A-2002-6257, illuminating light emitted from an illuminating optical system is totally reflected to DMD by a total reflection face of a total reflection prism. DMD modulates illuminating light into image light and image light from DMD is incident on a projecting optical system by transmitting the total reflection face. Therefore, the image light transmits through an air gap of the total reflection prism. The lower the parallelism of faces of the two triangular prisms opposed to each other via the air gap, the more produced is the aberration and therefore, it is necessary to make the two faces in parallel with each other with a high accuracy. That is, in the total reflection prism, a high positional accuracy is requested between the two triangular prisms in addition to an accuracy of shapes of the respective triangular prisms.
According to a projecting display apparatus described in JP-A-2003-262918, illuminating light emitted from an illuminating optical system is incident on DMD by transmitting through a total reflection prism. Image light modulated by DMD is totally reflected to a projecting optical system by a total reflection face of the total reflection prism. In this case, image light is not transmitted through an air gap between two triangular prisms and therefore, so far as an accuracy of a shape of a triangular prism (plane accuracy of reflecting face or the like) satisfies constant level, even when a positional accuracy of the two triangular prisms is comparatively low, a grade of a projected image is not deteriorated and time and labor required in fabricating the total reflection prism can be alleviated.
Further, the projecting display apparatus described in JP-A-2003-262918 is provided with the illuminating optical system and the optical projecting system such that optical axes thereof are substantially in parallel with each other and is provided with a circuit board with a control circuit for driving DMD such that a board face thereof becomes substantially in parallel with the optical axes of the respective optical systems. Thereby, the circuit board is restrained from being projected in comparison with the case of arranging the circuit board perpendicularly to the optical axes of the respective optical systems and large-sized formation of the projector engine is prevented.
However, when the illuminating optical system and the projecting optical system are provided such that optical axes thereof become perpendicular to each other to adapt to projection of a rear system different from that described in JP-A-2003-262918, there poses problem that according to a total reflection prism arranged between the illuminating optical system and the projecting optical system, from a necessity of ensuring a critical angle for totally reflecting image light, an incident direction of illuminating light and an emitting direction of image light cannot be made to be perpendicular to each other and it is difficult to arrange the illuminating optical system and the projecting optical system and the total reflection prism two-dimensionally on the same plane. The invention has been carried out in consideration of the above-described problem